Coatings are effective for improving the performance of various materials, such as for achieving better wear resistance and corrosion resistance. Common applications where a coating is applied to a substrate to improve wear resistance of the substrate material include cutting tool inserts for the cutting of hard materials, such as steel. Common substrate materials for cutting tools may include, for example, steel, other hard metals, and ceramics. The desired outer coating for the substrate may be of a material that does not bond well with the material of which the substrate is formed. Due to thermal expansion differences, typical coatings of materials such as titanium carbide, titanium carbonitride, titanium nitride, zirconia carbonitride and aluminum oxide do not adhere well to ceramic substrates, especially silicon nitrides and aluminum oxides.
Many coatings using these materials are produced at high temperature, which results in eta phases in the coating. Eta phases are a carbon-deficient form of a carbide material that result in a harder, much more brittle cemented carbide structure, which due to its brittle properties is prone to breakage. This breakage reduces the wear life of the cutting tool. An interfacial layer may be used to seal the substrate and prevent the formation of eta phase material. An adherent layer of a different material that is positioned as an interfacial layer between the substrate and the outer coating(s) may therefore be desired.
Hafnium nitride (HfN) has been explored as a coating material in the development of cutting tools. A desirable property of HfN in such applications is that it tends to “bend” rather than fracture under high-temperature conditions. It thus exhibits a good resistance to thermal cracking during cutting tool use. U.S. Pat. No. 4,406,667 to Sarin et al. teaches a composite substrate having a coating of a refractory metal nitride, including hafnium nitride as one example coating material.
Despite its resistance to thermal cracking, HfN is relatively soft when compared to other known cutting tool coating materials, and thus quickly wears away if used as the only coating on a cutting tool substrate. Cutting tool inserts using HfN as the only substrate coating layer or as the outer substrate coating layer are thus not widely used. U.S. Pat. No. 6,447,890 to Leverenz et al. teaches a hard metal substrate with two coatings, the first coating in contact with the substrate being of various potential materials, including a metal nitride of a group of metals including hafnium. The second coating may be a metal carbide, metal nitride, or metal oxide, including aluminum oxide (Al2O3). Thus Leverenz et al. '890 teaches HfN as a potential inner layer material with a harder material used as an overcoat or outer layer. Leverenz et al. '890 teaches, however, that the HfN first coating should be at least about 2 microns in thickness, preferably at least about 2 microns to 5 microns in thickness. Leverenz et al. '890 teaches that this innermost layer is provided to inhibit thermal cracking, and further that the specific thickness of at least 2 microns for the inner HfN layer is preferred so as to reduce the occurrence of cracks during the cutting process, which can lead to failure.
While HfN used as an interfacial layer does provide resistance to cracking for cutting tool inserts, the application of a thick HfN coating as taught by Leverenz et al. '890 significantly increases production costs for cutting tool inserts. Hafnium is itself a rare and expensive material, and the processes required to apply a coating in the thickness range specified by Leverenz et al. '890 require a substantial amount of coating time to achieve. Thus the time to produce each cutting tool insert is significantly increased, thereby lowering the overall manufacturing efficiency associated with the production of such cutting tool inserts, and significantly increasing their cost. For this reason, a cutting tool with improved resistance to thermal cracking, but that does not require the increased production time associated with a thick HfN interfacial layer, is highly desired.
References mentioned in this background section are not admitted to be prior art with respect to the present invention.